302 LAND Mammals in the western hemisphere 



The Bridger horses are fortunately much better known. 

 There are several species of the genus ^Orohippus, which form 

 a connected and progressive series ; and, though much smaller 

 than the smallest and oldest of the White River forms, they were 

 somewhat larger than the known representative of the Uinta, 

 ^Epihippus, but distinctly more primitive in all other respects. 

 The incisors were simple cutting teeth, with no trace of even 

 an incipient "mark," and only one premolar in each jaw, the 

 hindmost one, had taken on the molar-pattern. The orbit 

 was farther forward in the skull and less enclosed behind than 

 in ^Mesohippus, the cranium narrower and less capacious ; 

 the neck was even shorter and the odontoid process of the 

 axis still retained the primitive peg-like form. The Umbs and 

 feet were conspicuously shorter in proportion than those of 

 the White River genus ; the ulna and fibula were stouter and 

 less reduced and entirely separate from the radius and tibia 

 respectively. The front foot had four functional toes; the 

 fifth digit, which in ^Mesohippus had been reduced to a spUnt, 

 was completely developed in the Bridger horses, but the hind 

 foot was three-toed. 



Passing over, for lack of space, the transitional forms of 

 the Wind River, we come finally to the most ancient known 

 horses, the Wasatch species comprised in the genus ^EoMppxis, 

 the "Dawn Horse," as its name signifies; these were Uttle 

 creatures ranging in size from a cat to a small fox. Despite 

 an unmistakably equine look in the skeletons of these di- 

 minutive animals, it is only the long intermediate series of 

 species and genera, together forming a closely linked chain, 

 which we have traced back from the Pleistocene to the lower 

 Eocene, that leads us to regard ^Eohippus as the ancestral 

 type of the horses. Were only the two ends of the chain 

 known, he would be a daring speculator who should venture 

 to connect them. In these little Wasatch horses we have 

 the evidence of ^ a still more ancient form with five fully 

 developed toes in each foot, since the front foot had four 



